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Re: Output transformer roasted?

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3/26/2004 6:47 AM

Re: Output transformer roasted?

I suppose if you wanted to test the transformer empirically you could disconnect the leads and measure the resistance of each side of the primary with a DMM. The values will be very low but should be more or less equal.

You could then reconnect the transformer swapping the leads and see if the problem also moves to the other side. Make sure to disconnect any feedback if you reverse the leads for testing.

-s

3/26/2004 12:58 PM
Wild Bill	AAron, you didn't say if you had swapped pairs of tubes to see if the problem followed the tubes or stayed with the socket. You could simply have a weak pair on one side. Did you measure the bias individually on each tube or the whole schmozzle? ---Wild Bill

3/26/2004 4:33 PM

Sometimes it can be just due to uneven cooling. If the power tubes are old, or the grid leak resistors have drifted high, the tubes could be on the edge of runaway, and if one side is slightly hotter to start with, it'll run away first. Once one side runs away and starts to glow red, it sucks all the power, so the other side won't run away.

I had an amp like this, where one tube would run away and start to red plate, but only when the amp was stood vertically on its end on the bench. The runaway tube was in the rising hot air from the other tube.

Steve C.

3/26/2004 6:26 PM

Well, I did try swapping tubes and well as trying a whole new (well, actually old) set of tubes. It stays with the two tube sockets. I measured the DC resistance on both sides of the primary and found them to be equal. I measured the bias on each tube when it was idle, and all the currents seemed to be more or less equal.

I think I may try the output transformer primary lead swap just to rule the transformer out ... I will also check the grid leak resistors.

-aaron

3/26/2004 9:39 PM

I wonder if there could be a problem at the input of the PI. If it was seriously unbalanced you could be driving one output side much harder than the other.

Until you get that scope you could measure whatever you can at the PI input. You could check the voltage drop across the plate resistors and see if they're what you'd expect if the stage was working, or not working for that matter.

I once had a V4B in that had a weird problem with the PI tube itself. It checked out fine in my tester but actually had no plate connection on one of the PI triodes. I found it by noticing there was no voltage drop across its plate resistor, which meant the tube wasn't drawing any current so the voltage was the same at both ends. The symptom was similar - redplating on one side only and kinda harsh tone.

Just a thought...

---Wild Bill

3/29/2004 10:40 AM

quote:
"It stays with the two tube sockets."

Yup, it sounds like the same "problem" I had.

4/6/2004 1:34 PM

Ok.. more on this saga... it's not the output transformer, I swapped the leads and disconnected the NFB and it was the same 2 tubes that begin to redplate.

So I bust out a scope and measure the grids of the power tubes. On the side the starts to glow, when the amp is maxed I get kind of a square wave and half of the cycle is at 0 volts. The other pair of tubes does the same, but it doesn't stay at 0 volts for as long. So there's a problem there.

I also read the bias voltage from where the bias feed resistors connect, in other words not the grid connected side of the bias feed, but the side where it connects to the bias circuit. When I play with the volume cranked, the bias voltage at that point goes MORE NEGATIVE. It should stay the same regardless.. shouldn't it? Or am I just missing something here?

thanks,
-aaron

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